diff --git a/app/src/main/java/jp/juggler/subwaytooter/util/AttachmentRequest.kt b/app/src/main/java/jp/juggler/subwaytooter/util/AttachmentRequest.kt
index 98158c78..608d5e05 100644
--- a/app/src/main/java/jp/juggler/subwaytooter/util/AttachmentRequest.kt
+++ b/app/src/main/java/jp/juggler/subwaytooter/util/AttachmentRequest.kt
@@ -3,6 +3,8 @@ package jp.juggler.subwaytooter.util
 import android.content.Context
 import android.graphics.Bitmap
 import android.net.Uri
+import jp.juggler.media.generateTempFile
+import jp.juggler.media.transcodeAudio
 import jp.juggler.subwaytooter.R
 import jp.juggler.subwaytooter.api.entity.InstanceType
 import jp.juggler.subwaytooter.api.entity.TootInstance
@@ -32,14 +34,26 @@ class AttachmentRequest(
 ) {
     companion object {
         private val log = LogCategory("AttachmentRequest")
-    }
 
-    fun hasServerSupport(mimeType: String) =
-        mediaConfig?.jsonArray("supported_mime_types")?.contains(mimeType)
-            ?: when (instance.instanceType) {
-                InstanceType.Pixelfed -> AttachmentUploader.acceptableMimeTypesPixelfed
-                else -> AttachmentUploader.acceptableMimeTypes
-            }.contains(mimeType)
+        private val goodAudioType = setOf(
+            "audio/flac",
+            "audio/mp3",
+            "audio/ogg",
+            "audio/vnd.wave",
+            "audio/vorbis",
+            "audio/wav",
+            "audio/wave",
+            "audio/webm",
+            "audio/x-pn-wave",
+            "audio/x-wav",
+            "audio/3gpp",
+        )
+//    val badAudioType = setOf(
+//        "audio/mpeg","audio/aac",
+//        "audio/m4a","audio/x-m4a","audio/mp4",
+//        "video/x-ms-asf",
+//    )
+    }
 
     suspend fun createOpener(): InputStreamOpener {
 
@@ -122,39 +136,50 @@ class AttachmentRequest(
         )
     }
 
+    private fun hasServerSupport(mimeType: String) =
+        mediaConfig?.jsonArray("supported_mime_types")?.contains(mimeType)
+            ?: when (instance.instanceType) {
+                InstanceType.Pixelfed -> AttachmentUploader.acceptableMimeTypesPixelfed
+                else -> AttachmentUploader.acceptableMimeTypes
+            }.contains(mimeType)
+
     private fun createResizedImageOpener(
         forcePng: Boolean = false,
     ): InputStreamOpener {
-        val cacheDir = context.externalCacheDir
-            ?.apply { mkdirs() }
-            ?: error("getExternalCacheDir returns null.")
-
-        val outputMimeType = if (forcePng || mimeType == AttachmentUploader.MIME_TYPE_PNG) {
-            AttachmentUploader.MIME_TYPE_PNG
-        } else {
-            AttachmentUploader.MIME_TYPE_JPEG
-        }
-
-        val tempFile = File(cacheDir, "tmp." + Thread.currentThread().id)
-        val bitmap = createResizedBitmap(
-            context,
-            uri,
-            imageResizeConfig,
-            skipIfNoNeedToResizeAndRotate = !forcePng,
-            serverMaxSqPixel = serverMaxSqPixel
-        ) ?: error("createResizedBitmap returns null.")
-        pa.progress = context.getString(R.string.attachment_handling_compress)
+        val tempFile = context.generateTempFile("createResizedImageOpener")
         try {
-            FileOutputStream(tempFile).use { os ->
-                if (outputMimeType == AttachmentUploader.MIME_TYPE_PNG) {
-                    bitmap.compress(Bitmap.CompressFormat.PNG, 100, os)
-                } else {
-                    bitmap.compress(Bitmap.CompressFormat.JPEG, 95, os)
+            pa.progress = context.getString(R.string.attachment_handling_compress)
+
+            val bitmap = createResizedBitmap(
+                context,
+                uri,
+                imageResizeConfig,
+                skipIfNoNeedToResizeAndRotate = !forcePng,
+                serverMaxSqPixel = serverMaxSqPixel
+            ) ?: error("createResizedBitmap returns null.")
+            try {
+                val outputMimeType = when {
+                    forcePng || mimeType == AttachmentUploader.MIME_TYPE_PNG ->
+                        AttachmentUploader.MIME_TYPE_PNG
+
+                    else -> AttachmentUploader.MIME_TYPE_JPEG
                 }
+                FileOutputStream(tempFile).use { outStream ->
+                    when (outputMimeType) {
+                        AttachmentUploader.MIME_TYPE_PNG ->
+                            bitmap.compress(Bitmap.CompressFormat.PNG, 100, outStream)
+
+                        else ->
+                            bitmap.compress(Bitmap.CompressFormat.JPEG, 95, outStream)
+                    }
+                }
+                return tempFileOpener(tempFile, outputMimeType, isImage = true)
+            } finally {
+                bitmap.recycle()
             }
-            return tempFileOpener(tempFile, outputMimeType, isImage = true)
-        } finally {
-            bitmap.recycle()
+        } catch (ex: Throwable) {
+            tempFile.delete()
+            throw ex
         }
     }
 
@@ -168,13 +193,13 @@ class AttachmentRequest(
         val outFile = File(cacheDir, "movie." + Thread.currentThread().id + ".mp4")
         var resultFile: File? = null
 
-        // 入力ファイルをコピーする
-        (context.contentResolver.openInputStream(uri)
-            ?: error("openInputStream returns null.")).use { inStream ->
-            FileOutputStream(tempFile).use { inStream.copyTo(it) }
-        }
-
         try {
+            // 入力ファイルをコピーする
+            (context.contentResolver.openInputStream(uri)
+                ?: error("openInputStream returns null.")).use { inStream ->
+                FileOutputStream(tempFile).use { inStream.copyTo(it) }
+            }
+
             // 動画のメタデータを調べる
             val info = tempFile.videoInfo
 
@@ -224,27 +249,34 @@ class AttachmentRequest(
         }
     }
 
-    private val aacMimeTypes = listOf(
-        "audio/aac", "audio/aacp", "audio/3gpp", "audio/3gpp2",
-        "audio/mp4", "audio/MP4A-LATM", "audio/mpeg4-generic"
-    ).map { it.lowercase() }.toSet()
-
-    private fun createResizedAudioOpener(
-        originalBytes: Long,
-    ): InputStreamOpener {
-        if (hasServerSupport("audio/aac") && aacMimeTypes.contains(mimeType.lowercase())) {
-            mimeType = "audio/aac"
-        }
-
-        // サーバ側がサポートしてる形式でサイズ以内なら
-        if (hasServerSupport(mimeType) && originalBytes <= maxBytesVideo) {
-            return contentUriOpener(
+    private suspend fun createResizedAudioOpener(srcBytes: Long): InputStreamOpener =
+        when {
+            hasServerSupport(mimeType) &&
+                    goodAudioType.contains(mimeType) &&
+                    srcBytes <= maxBytesVideo -> contentUriOpener(
                 context.contentResolver,
                 uri,
                 mimeType,
                 isImage = false,
             )
+
+            else -> {
+                pa.progress = context.getString(R.string.attachment_handling_compress)
+                val (tempFile, outMimeType) = transcodeAudio(
+                    context,
+                    uri,
+                    mimeType
+                )
+                // このワークアラウンドはうまくいかなかった
+//                outMimeType = when (outMimeType) {
+//                    "audio/mp4" -> "audio/x-m4a"
+//                    else -> outMimeType
+//                }
+                tempFileOpener(
+                    tempFile,
+                    outMimeType,
+                    isImage = false,
+                )
+            }
         }
-        error("audio conversion is not yet supported.")
-    }
 }
diff --git a/app/src/main/java/jp/juggler/subwaytooter/util/AttachmentUploader.kt b/app/src/main/java/jp/juggler/subwaytooter/util/AttachmentUploader.kt
index ec83f245..b844bbd5 100644
--- a/app/src/main/java/jp/juggler/subwaytooter/util/AttachmentUploader.kt
+++ b/app/src/main/java/jp/juggler/subwaytooter/util/AttachmentUploader.kt
@@ -6,6 +6,7 @@ import android.os.Handler
 import android.os.SystemClock
 import androidx.annotation.WorkerThread
 import androidx.appcompat.app.AppCompatActivity
+import androidx.media3.common.MimeTypes
 import jp.juggler.subwaytooter.R
 import jp.juggler.subwaytooter.api.TootApiCallback
 import jp.juggler.subwaytooter.api.TootApiClient
@@ -295,15 +296,15 @@ class AttachmentUploader(
             val isAccepteble = instance.configuration
                 ?.jsonObject("media_attachments")
                 ?.jsonArray("supported_mime_types")
-                ?.contains(mimeType)
+                ?.contains(opener.mimeType)
                 ?: when (instance.instanceType) {
                     InstanceType.Pixelfed -> acceptableMimeTypesPixelfed
                     else -> acceptableMimeTypes
-                }.contains(mimeType)
+                }.contains(opener.mimeType)
 
             if (!isAccepteble) {
                 return TootApiResult(
-                    safeContext.getString(R.string.mime_type_not_acceptable, mimeType)
+                    safeContext.getString(R.string.mime_type_not_acceptable, opener.mimeType)
                 )
             }
 
diff --git a/base/build.gradle b/base/build.gradle
index 709ef386..3a9f324b 100644
--- a/base/build.gradle
+++ b/base/build.gradle
@@ -98,7 +98,9 @@ dependencies {
     api "org.jetbrains.kotlinx:kotlinx-datetime:0.4.0"
     api "org.jetbrains.kotlinx:kotlinx-serialization-json:1.4.1"
     api "ru.gildor.coroutines:kotlin-coroutines-okhttp:1.0"
-
+    api "androidx.media3:media3-transformer:1.0.1"
+    api "androidx.media3:media3-effect:1.0.1"
+    api "androidx.media3:media3-common:1.0.1"
     // commons-codecをapiにすると、端末上の古いjarが使われてしまう
     // declaration of 'org.apache.commons.codec.binary.Base64' appears in /system/framework/org.apache.http.legacy.jar)
     androidTestImplementation "commons-codec:commons-codec:1.15"
diff --git a/base/src/main/java/jp/juggler/media/AudioTranscoder.kt b/base/src/main/java/jp/juggler/media/AudioTranscoder.kt
new file mode 100644
index 00000000..4a4f16e6
--- /dev/null
+++ b/base/src/main/java/jp/juggler/media/AudioTranscoder.kt
@@ -0,0 +1,112 @@
+package jp.juggler.media
+
+import android.content.Context
+import android.net.Uri
+import android.os.Looper
+import androidx.media3.common.MediaItem
+import androidx.media3.common.MimeTypes
+import androidx.media3.transformer.TransformationException
+import androidx.media3.transformer.TransformationRequest
+import androidx.media3.transformer.TransformationResult
+import androidx.media3.transformer.Transformer
+import jp.juggler.util.log.LogCategory
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.ExperimentalCoroutinesApi
+import kotlinx.coroutines.suspendCancellableCoroutine
+import kotlinx.coroutines.withContext
+import java.io.File
+import java.io.FileOutputStream
+import java.nio.file.Files
+import java.nio.file.Paths
+import java.util.UUID
+import kotlin.coroutines.resumeWithException
+
+private val log = LogCategory("transcodeAudio")
+
+val generateTempFileLock = Any()
+
+fun Context.generateTempFile(prefix: String) =
+    synchronized(generateTempFileLock) {
+        val cacheDir = externalCacheDir ?: cacheDir ?: error("missing cacheDir")
+        cacheDir.mkdirs()
+        val path = Paths.get(cacheDir.canonicalPath)
+        if (!Files.isDirectory(path)) error("cacheDir is not directory. $cacheDir")
+        if (!Files.isWritable(path)) error("cacheDir is not writable. $cacheDir")
+        // 重複しない一時ファイル名を探す
+        var tempFile: File
+        do {
+            tempFile = File(cacheDir, "$prefix-${UUID.randomUUID()}")
+        } while (tempFile.exists())
+        // すぐにファイルを作成する
+        FileOutputStream(tempFile).use {}
+        tempFile
+    }
+
+@OptIn(ExperimentalCoroutinesApi::class)
+@androidx.annotation.OptIn(androidx.media3.common.util.UnstableApi::class)
+suspend fun transcodeAudio(
+    context: Context,
+    inUri: Uri,
+    inMimeType: String,
+): Pair<File, String> {
+    val inputMediaItem = MediaItem.fromUri(inUri)
+    // トランスコードに指定するmimeType
+    val encodeMimeType = MimeTypes.AUDIO_AAC
+    // MediaStore登録に使うmimeType。 audio/mp4a-latm だと失敗するのホント困る
+    val storeMimeType = "audio/mp4"
+    val tmpFile = context.generateTempFile("transcodeAudio")
+
+    // Transformerは単一スレッドで処理する要件
+    val result: TransformationResult = withContext(Dispatchers.Main.immediate) {
+        val looper = Looper.getMainLooper()
+        suspendCancellableCoroutine { cont ->
+            val transformerListener = object : Transformer.Listener {
+                override fun onTransformationCompleted(
+                    inputMediaItem: MediaItem,
+                    transformationResult: TransformationResult,
+                ) {
+                    log.i("onTransformationCompleted inputMediaItem=$inputMediaItem transformationResult=$transformationResult")
+                    if (cont.isActive) cont.resume(transformationResult) {}
+                }
+
+                override fun onTransformationError(
+                    inputMediaItem: MediaItem,
+                    exception: TransformationException,
+                ) {
+                    log.e(
+                        exception,
+                        "onTransformationError inputMediaItem=$inputMediaItem"
+                    )
+                    if (cont.isActive) cont.resumeWithException(exception)
+                }
+
+                override fun onFallbackApplied(
+                    inputMediaItem: MediaItem,
+                    originalTransformationRequest: TransformationRequest,
+                    fallbackTransformationRequest: TransformationRequest,
+                ) {
+                    log.i("onFallbackApplied inputMediaItem=$inputMediaItem original=$originalTransformationRequest fallback=$fallbackTransformationRequest")
+                }
+            }
+            val transformer = Transformer.Builder(context)
+                .setLooper(looper)
+                .setTransformationRequest(
+                    TransformationRequest.Builder()
+                        .setAudioMimeType(encodeMimeType)
+                        .build()
+                )
+                .setRemoveVideo(true)
+                .setRemoveAudio(false)
+                .addListener(transformerListener)
+                .build()
+            transformer.startTransformation(inputMediaItem, tmpFile.canonicalPath)
+            cont.invokeOnCancellation {
+                transformer.cancel()
+            }
+        }
+    }
+    result.run {
+        log.i("result: durationMs=$durationMs, fileSizeBytes=$fileSizeBytes, averageAudioBitrate=$averageAudioBitrate, averageVideoBitrate=$averageVideoBitrate, videoFrameCount=$videoFrameCount")
+    }
+    return Pair(tmpFile, storeMimeType)
+}